Synthesis and characterization of scVEGF‐PEG‐[68Ga]NOTA and scVEGF‐PEG‐[68Ga]DOTA PET tracers

Vascular endothelial growth factor (VEGF) signaling via vascular endothelial growth factor receptor 2 (VEGFR‐2) on tumor endothelial cells is a critical driver of tumor angiogenesis. Novel anti‐angiogenic drugs target VEGF/VEGFR‐2 signaling and induce changes in VEGFR‐2 prevalence. To monitor VEGFR‐2 prevalence in the course of treatment, we are evaluating ⁶⁸Ga positron emission tomography imaging agents based on macrocyclic chelators, site‐specifically conjugated via polyethylene glycol (PEG) linkers to engineered VEGFR‐2 ligand, single‐chain (sc) VEGF. The ⁶⁸Ga‐labeling was performed at room temperature with NOTA (2,2′,2′′‐(1,4,7‐triazonane‐1,4,7‐triyl) triacetic acid) conjugates or at 90 °C by using either conventional or microwave heating with NOTA and DOTA (2,2′,2′′,2′′′‐(1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrayl) tetraacetic acid) conjugates. The fastest (~2 min) and the highest incorporation (>90%) of ⁶⁸Ga into conjugate that resulted in the highest specific radioactivity (~400 MBq/nmol) was obtained with microwave heating of the conjugates. The bioactivity of the NOTA‐ and DOTA‐containing tracers was validated in 3‐D tissue culture model of 293/KDR cells engineered to express high levels of VEGFR‐2. The NOTA‐containing tracer also displayed a rapid accumulation (~ 20 s after intravenous injection) to steady‐state level in xenograft tumor models. A combination of high specific radioactivity and maintenance of functional activity suggests that scVEGF‐PEG‐[⁶⁸ Ga]NOTA and scVEGF‐PEG‐[⁶⁸ Ga]DOTA might be promising tracers for monitoring VEGFR‐2 prevalence and should be further explored. Copyright © 2011 John Wiley & Sons, Ltd.